Null Values

Lua tables cannot store nil as a value - assigning nil to a key removes it from the table. SIMPLOO provides the null keyword to handle this.

The Problem

In plain Lua, you can't store nil in a table:

local t = { value = nil }
print(t.value)  -- nil
print(t["value"])  -- nil

-- But the key doesn't exist:
for k, v in pairs(t) do
    print(k, v)  -- prints nothing
end

This causes problems when you want a class member to default to nil:

-- This doesn't work as expected
class "Example" {
    data = nil;  -- This key won't exist in the class definition
}

The Solution: null

Use null to explicitly set a member's default value to nil:

Block SyntaxBuilder Syntax

class "Player" {
    name = "Unknown";
    guild = null;  -- Will be nil by default
    pet = null;    -- Will be nil by default
}

local player = class("Player")
player.name = "Unknown"
player.guild = null  -- Will be nil by default
player.pet = null    -- Will be nil by default
player:register()

local p = Player.new()
print(p.name)   -- Unknown
print(p.guild)  -- nil
print(p.pet)    -- nil

Common Use Cases

Optional References

class "Employee" {
    name = "";
    manager = null;  -- May or may not have a manager

    setManager = function(self, mgr)
        self.manager = mgr
    end;

    getManagerName = function(self)
        if self.manager then
            return self.manager.name
        end
        return "No manager"
    end;
}

local alice = Employee.new()
alice.name = "Alice"

local bob = Employee.new()
bob.name = "Bob"
bob:setManager(alice)

print(alice:getManagerName())  -- No manager
print(bob:getManagerName())    -- Alice

Lazy Initialization

class "DataLoader" {
    cache = null;  -- Not loaded yet

    get = function(self)
        if self.cache == nil then
            self.cache = self:loadFromDisk()
        end
        return self.cache
    end;

    loadFromDisk = function(self)
        print("Loading data...")
        return { "item1", "item2", "item3" }
    end;
}

local loader = DataLoader.new()
print(loader.cache)  -- nil
loader:get()         -- Loading data...
print(#loader.cache) -- 3
loader:get()         -- (no output, already loaded)

What is null?

null is a special marker value that SIMPLOO recognizes. During class registration, any member with value null is converted to nil.

print(null)  -- NullVariable_WgVtlrvpP194T7wUWDWv2mjB

Note

Never compare values against null at runtime. Use nil for comparisons:

-- Correct
if self.data == nil then

-- Wrong (will never be true for actual nil values)
if self.data == null then

Complete Example

Block SyntaxBuilder Syntax

dofile("simploo.lua")

class "TreeNode" {
    value = 0;
    left = null;
    right = null;

    __construct = function(self, val)
        self.value = val
    end;

    insert = function(self, val)
        if val < self.value then
            if self.left == nil then
                self.left = TreeNode.new(val)
            else
                self.left:insert(val)
            end
        else
            if self.right == nil then
                self.right = TreeNode.new(val)
            else
                self.right:insert(val)
            end
        end
    end;

    printInOrder = function(self)
        if self.left then self.left:printInOrder() end
        print(self.value)
        if self.right then self.right:printInOrder() end
    end;
}

local root = TreeNode.new(5)
root:insert(3)
root:insert(7)
root:insert(1)
root:insert(9)

root:printInOrder()
-- Output: 1, 3, 5, 7, 9

dofile("simploo.lua")

local node = class("TreeNode")
node.value = 0
node.left = null
node.right = null

function node:__construct(val)
    self.value = val
end

function node:insert(val)
    if val < self.value then
        if self.left == nil then
            self.left = TreeNode.new(val)
        else
            self.left:insert(val)
        end
    else
        if self.right == nil then
            self.right = TreeNode.new(val)
        else
            self.right:insert(val)
        end
    end
end

function node:printInOrder()
    if self.left then self.left:printInOrder() end
    print(self.value)
    if self.right then self.right:printInOrder() end
end

node:register()

local root = TreeNode.new(5)
root:insert(3)
root:insert(7)
root:insert(1)
root:insert(9)

root:printInOrder()
-- Output: 1, 3, 5, 7, 9